Effects of nerve growth factor on differentiation of muscle spindles following nerve lesion in neonatal rats.

Central synaptic function of Group Ia sensory fibers has been shown to be enhanced by daily applications of NGF in neonatal rats. In the periphery, Group Ia sensory fibers are known to trigger the differentiation of muscle spindles. In the present study we examined, in neonatal rats, whether daily NGF treatment affects the number or structure of spindles formed in muscle whose nerve has been crushed. The results show that 25-35 d after nerve crush, de novo supernumerary spindles were formed by treatment with NGF in the temporarily denervated muscle. The number of spindles in the corresponding muscle on the contralateral, intact side was not affected by NGF treatment. Following nerve crush alone, the mean number of spindles in the muscle was not significantly different from the normal value. It is suggested that treatment of neonatal rats with NGF may facilitate the outgrowth of excessive peripheral collaterals of regenerating Group Ia sensory fibers, which, in turn, contributes to the formation of de novo spindles as a result of interaction with myotubes.